Numerous infectious diseases are caused by minority strains within otherwise nonpathogenic species, but the ecological and population genetic mechanisms that drive their emergence remains unclear. Understanding these dynamics is especially important in light of the increasing frequency of many epidemics tied to changes in climate and land use. For example, illness caused by contaminated shellfish is relatively infrequent in most of the U.S. but incidence is on the rise and beginning to parallel that of warmer countries. The abundance of the major causative bacterium Vibrio parahaemolyticus (Vp) is positively correlated with warmer waters with moderate salinity. Yet these ecological variables do not sufficiently predict Vp epidemics in part because the relative abundance of potentially pathogenic Vp is far less predictable. This project seeks to identify and quantify the ecological factors that influence the relative prevalence of virulent Vp in oysters. Our model study area, the Great Bay estuary of New Hampshire, is ideal because of the wide range of environmental conditions that occur over relatively small spatial scales, and because its warmer waters promote species typically associated with warmer climates. Vp are known to reside in this estuary along with other pathogenic Vibrios and there have been several acute Vp infections from the estuary suggesting that minority pathogenic strains are resident in the community. Our focal question is: how do environmental factors drive the population dynamics of Vp? We hypothesize that Vp strains that vary in virulence traits are associated with certain environmental variables, such that fluctuations in climate will specifically influence the relative abundance of virulent Vp. We base this hypothesis on our preliminary data that shows a significant correlation between the prevalence of highly cytotoxic strains and increased temperature. Using multi-locus sequence typing analysis (MLST), we will examine the population structure and extent of recombination between estuary isolates. Our preliminary genetic typing thus far suggests that there is significant genetic exchange within the community that has three distinct Vp populations. The proposed study is unique from those published, specifically by 1) the inclusion of the diverse natural population (not only a subset with specific virulence markers), 2) a genetic typing scheme that includes known and putative virulence genes and allows the detection of virulence gene transfer within the population, and 3) it includes a characterization of functional virulence that will be overlaid upon the molecular typing scheme. We have two specific aims that address the following questions: Do environmental factors influence the virulence potential of natural Vp populations in oysters? What is the genetic structure of northern Vp populations and how are pathogenic elements linked to this structure? PUBLIC HEALTH RELEVANCE: Recent major outbreaks of Vibrio parahaemolyticus in cooler northern climes have been linked not to the pandemic clone, but to distinct resident bacteria of unique genetic composition, perhaps foreshadowing the increasing risk to human health due to global climate change. A major obstacle to assessing risk of infection and developing preventative protocols is a lack of understanding of what factors contribute to the emergence of virulent biovars within natural V. parahaemolyticus populations. The combined molecular typing and virulence assessment in the context of estuarine ecology will provide invaluable insight necessary to generate predictive models for this emerging pathogen.